1. Field of the Invention
This invention relates to an image forming apparatus such as a copying machine, a printer or a facsimile apparatus for effecting image forming by an electrophotographic process, an electrostatic recording process or the like.
2. Related Background Art
In a well-known image forming apparatus including the step of transferring a toner image formed on the surface of an image bearing member to a transferring material such as paper, there is put into practical use one designed such that the transferring material is passed to a transferring region formed on the portion of contact between the image bearing member and a transferring member such as a transferring roller brought into pressure contact therewith, and in timed relationship therewith, a transferring bias is applied to the transferring member, and by the action of an electric field formed by the applied transferring bias, the toner image on the surface of the image bearing member is shifted to the transferring material by the action of an electric field formed by the applied transferring bias.
The transferring roller has its resistance value adjusted to a value of the order of 1×106-1×1010 (Ω), but a transferring roller proposed in recent years, as shown in FIG. 3 of the accompanying drawings, has an elastic layer 118 provided on the outer peripheral surface of an electrically conductive mandrel 117, and this elastic layer 118 is given electrical conductivity. The transferring roller 116 is broadly classified into the following two kinds by the manner in which the elastic layer is given this electrical conductivity.                a transferring roller having a material of an electron electrically conducting system, and        a transferring roller having a material of an ion electrically conducting system.        
The above-mentioned transferring roller, as shown in FIG. 3, has an elastic layer 118, and an electrically conductive filler is dispersed in this elastic layer 118, and as an example, mention can be made of an EPDM roller or a urethane roller having an electrically conductive filler such as carbon or a metal oxide dispersed therein.
Alternatively, mention may be made of a material including a material of the ion electrically conducting system in the elastic layer 118, for example, a material itself such as urethane given electrical conductivity, or an interfacial active agent dispersed in the elastic layer 118.
Also, it is known that the resistance of the transferring roller is liable to fluctuate in conformity with the temperature and humidity of the atmospheric environment, and it is feared that the fluctuation in the resistance of the transferring roller induces the arising of such problems as faulty transfer, explosive scatter and paper trace.
So, in order to prevent the occurrence of the faulty transfer and paper trace attributable to the fluctuation in the resistance of the transferring roller, there is adopted “applied transfer voltage control” measuring the resistance value of the transferring roller, and properly controlling a transferring voltage applied to the transferring roller in conformity with the result of the measurement.
A popular controlling method will be described later.
Now, in the transferring roller, there exists resistance unevenness in the direction of rotation thereof (hereinafter referred to as the “periphery unevenness”). This periphery unevenness becomes remarkable not only due to the non-uniformity of a roller resistance adjusting material, but also by being affected by partial changes in temperature and humidity. Specifically, it is the resistance difference by the temperature of a fixing apparatus between the region of the transferring roller opposed to a fixing roller and a side opposite thereto.
For example, when an electric current corresponding to one full rotation of the transferring roller is measured when the control of a certain constant voltage has been effected, the surface opposed to the fixing apparatus falls in the resistance value of the roller due to a high temperature, and becomes great in a current value flowing when a constant voltage is applied, as compared with a region which has not yet been warmed.
In order to avoid the inconvenience due to such a phenomenon, constant current control has been conceived, but for the following reason of phenomenon, constant voltage control is generally used.
To obtain a good transferring property at all times, it is ideal to control a charge amount supplied to a transferring region at a predetermined value, and for example, it is conceivable to constant-current-controlling the transferring roller. In the transferring region, however, the load impedance of the transferring roller to a photosensitive drum differs between a portion in which the transferring material is present and a portion in which the transferring material is absent, and the load impedance becomes small in the portion wherein the transferring material is absent.
Therefore, the width over which the transferring roller is in contact with the surface of the photosensitive drum at the transferring region is changed by a change in the size of the transferring material used, whereby much current concentratedly flows into the portion wherein the transferring material is absent, and faulty transfer is caused in the portion wherein the transferring material is present.
In contrast, if the constant voltage control is used, the same degree of charge amount is always supplied from the transferring roller differing in resistance value to the transferring region and therefore, a method which will hereinafter be described has been proposed.
So, in order to prevent the occurrence of faulty transfer and paper trace attributable to the fluctuation in the resistance of the transferring roller, there is adopted “applied transfer voltage control” for measuring the resistance value (voltage-current characteristic) of the transferring roller, and properly controlling a transferring voltage applied to the transferring roller in conformity with the result of the measurement.
As such applied transfer voltage control means, there is active transfer voltage control (ATVC) disclosed in Japanese Patent Application Laid-Open No. H2-123385.
The ATVC is means for optimizing a voltage applied to the transferring roller during transfer, and prevents the occurrence of faulty transfer and paper trace. The above-described transfer voltage is such that during the pre-multiple rotation step of the image forming apparatus, a desired constant current is applied from the transferring roller to the photosensitive drum, and the then voltage value is held to thereby detect the resistance of the transferring roller, and during the transfer at the printing step, a constant voltage conforming to that resistance value is applied as a transfer voltage to the transferring roller.
Also, as other applied transfer voltage control, mention may be made of programmable transfer voltage control (PTVC) disclosed in Japanese Patent Application Laid-Open No. H5-181373.
The ATVC effects the detection of the resistance of the transferring roller by constant current control, whereas the PTVC effects it by constant voltage control alone and therefore, a circuit therefor is simplified and detection accuracy is improved.
Particularly describing, the PTVC has means for applying a constant voltage during the detection of the resistance of the transferring roller, and detecting an output current value flowing to the photosensitive drum at this time, and when this current value is far from a set value, a constant voltage for detection is varied and outputted and the control is effected through software so that the set value may be obtained.
FIG. 2 of the accompanying drawings shows the construction of the PTVC. In FIG. 2, a PWM signal (DA value) having a pulse width corresponding to a desired transfer output voltage is first outputted from the OUT terminal of a CPU 101. Actually, a transfer output voltage table (not shown) corresponding to the pulse width is memorized in the CPU 101. This PWM signal is made into DC (analog) by a low-pass filter 102, is amplified by an amplifier 103 and becomes a transfer voltage TV. Next, voltage-current conversion is effected, and a signal corresponding to a current IT flowing at this time is inputted to the IN terminal of the CPU 101 after DA conversion, and is detected in the CPU 101.
As described above, the constant voltage control judges from the corresponding table of the PWM value preset in the CPU 101 and the transfer output voltage and outputs the PWM signal of a pulse width corresponding to the desired voltage value.
To accurately detect the resistance of the transferring roller by the above-described PTVC, and determine an optimum applied transfer voltage, the average current value corresponding to one full rotation of the transferring roller is monitored from the aforedescribed periphery unevenness of the transferring roller at a plurality of voltage values, and a target current is obtained from the relational expression of the current and the voltages. The resistance of the transferring roller has voltage dependency and therefore, the setting of such a voltage value that a value approximate to the voltage applied during transfer is generated is required. Consequently, it is usual that the PTVC, etc. are effected during pre-rotation having a surplus of time when carrying out an image forming process. So, in order to prevent the occurrence of faulty transfer, paper trace, etc., attributable to the fluctuation in the resistance of the transferring roller, there is adopted the “applied transfer voltage control” for measuring the resistance value of the transferring roller, and properly controlling a transfer voltage applied to the transferring roller in conformity with the result of the measurement.
According to the PTVC using the above-described conventional method, however, a plurality of voltage values corresponding to one full rotation of the transferring roller are applied and therefore, a time required for the detection of one full rotation of the transferring roller becomes necessary in conformity with the number of voltage levels applied during pre-rotation.
In the latest copying machines, there is the tendency toward shortening the first copy time, and it is also necessary to shorten the time required for the above-described control. Further, a higher image of quality has been advanced, and it is required to always obtain an optimum image by transfer control, and it is necessary to effect the above-described optimum control. For that purpose, it is necessary to effect the detection of one full rotation in the above-described control at plural levels of bias values, and determine an accurate bias for obtaining a necessary transfer current, but it is against the above-described tendency to shorten the first copy time to effect the detection of one full rotation of the transferring roller a plurality of times.